The present invention relates to ultrasound imaging. In particular, ultrasound imaging using increased transmit powers is provided.
In U.S. Pat. Nos. 5,957,847, 6,110,120, and 6,306,095, contrast agent imaging uses two different transmit power levels. Contrast agent microspheres are destroyed by standard ultrasound imaging power levels, such as B-mode imaging transmissions. The standard transmit powers are adapted to avoid violation of current limitations on transmit power, such as mechanical index and thermal indices. Loss of correlation due to the destruction of contrast agents may provide for high contrast images. However, to allow contrast agents to build up, transmit levels are reduced from standard transmit levels between higher power transmissions. The reduced transmit levels provide images while minimizing destruction of the contrast agent.
Increased transmit voltage or pulse length may be provided for cardiac applications, such as shown in U.S. Pat. No. 6,210,335, the disclosure of which is incorporated herein by reference. The imaging frame rate is reduced to allow an increase in transmit power within current FDA (Food And Drug Administration) and IEC (International Electrotechnical Commission) limits of mechanical index, thermal index, and transducer temperature. In response to a trigger, one or two frames of data associated with the increased transmit voltage or pulse length are acquired. The resulting images may have an increased color flow sensitivity or deeper penetration. However, the reduction in frame rate may be undesirable in some situations.
U.S. Pat. No. 5,487,387 discloses transmitting high intensity acoustic energy to initiate acoustic streaming of fluids within a region, and U.S. Pat. No. 6,371,912 discloses transmitting high intensity acoustic energy to initiate palpation of tissue, also called ARFI (acoustic radiation force imaging). Receive signals responsive to the high intensity signals are used to detect the acoustically induced movement or streaming of fluid. Acoustically induced movement of tissue may be detected as well.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include methods and systems for improving ultrasound image quality. On demand transmission of unsustainably high power ultrasonic pulses is temporally or spatially interleaved with sustainable low power, zero power, or standard ultrasonic pulses. The duration of the unsustainably high power ultrasonic pulses is a function of a time-tissue temperature relationship. Unsustainably high power pulses provide better penetration, improve signal-to-noise ratio and allow higher imaging frequencies for difficult to image patients in any of various modes, such as B-mode, harmonic B-mode responsive to tissue, contrast agent modes, color flow modes, M-mode, mixed modes, or other modes. These features are especially desirable for difficult-to-image patients, where body habitus makes satisfactory image quality problematic.
Sustainable transmit power is defined as that which can be employed indefinitely for imaging. Its upper bound is limited by system power delivery capability, transducer temperature, skin temperature, internal tissue temperature and peak rarefactional pressure. The various temperatures reach equilibrium values and are assumed to be maintained for the duration of the ultrasound imaging diagnostic procedure. Regulatory temperature limits are set so as to not cause thermal damage to the patient.
Further aspects and advantages of the invention are discussed below in conjunction with preferred embodiments.